Nanotubes: Superhard, superstrong, super useful

Single-walled carbon nanotubes are so oxygen-sensitive that the absorption of even a few atoms of oxygen (shown in green) can change semiconducting tubes into conductors. Click here for more photos.

"Alex Zettl makes the most incredible devices you'll never
see - at least not without the aid of an electron microscope.
Zettl, a physicist who holds a joint appointment with
Berkeley Lab's Materials Sciences Division (MSD) and the
Physics Department of the University of California at
Berkeley, has led the creation of what may be the world's
smallest human-made bearings and mechanical switches,
the world's smallest room-temperature diodes, and a "tube
cube" electronic device with the potential to wire itself.
These devices have been fashioned from hollow tubular
macromolecules only a few nanometers (billionths of a
meter) in diameter. Such macromolecules are called
"nanotubes" and they have emerged as premier building
blocks for the coming age of nanotechnology.

Not only do nanotubes offer a full range of
electrical and thermal conductivity properties
(they conduct heat better than any other known
material), they're also about a hundred times
stronger than steel and more durable than
diamonds. Their potential for use in electronics is
nothing short of mind-boggling: if all the
nanotubes that could be packed into a
one-half-inch cube were to be laid out end to
end, they would stretch some 250,000 miles.

"The most exciting thing is that a lot of
structures we are now making in the laboratory
and studying are very relevant to everyday life,
from being used as structural materials, to
electronic materials, to chemical sensing," says
Zettl. "In almost any technological application
you want to think of, nanotubes probably will
have an impact."

Nanotubes are two-dimensional crystalline sheets of atoms that have been rolled up and
connected at the seam to form a closed cylinder. The earliest nanotubes were made from
pure carbon. Formed naturally in the sooty residue of vaporized carbon rods, they were
an elongated form of fullerene or "buckyball" molecules, clusters of 60 and 70 carbon
atoms joined in a graphite-like mesh of hexagonal rings. The first generations were
"multi-walled nanotubes" (MWNTs): about five to 40 single-walled nanotubes
(SWNTs)-meaning the tube's surface consists of only a single layer of carbon
atoms-each tube nesting inside the other like Russian dolls. Later, when scientists began
to directly make SWNTs, it was discovered that they could be drawn out to exceedingly
long lengths of nanowire without losing any strength or durability.

While the number of potential applications foreseen for nanotubes is large, it has long
been thought that the most valuable applications will be in electronics. In principle,
nanotubes could play a similar role to silicon for electronic devices but at a molecular
scale where silicon and other semiconductors cease to function. Zettl and his research
group became the first to demonstrate that electronic devices form naturally during the
creation of pure carbon nanotubes.

Depending upon its diameter or its chirality (geometric
configuration), a pure carbon nanotube can conduct an
electrical current as if it were a metal, or it can act as a
semiconductor, meaning it will only conduct a current that
exceeds a threshold voltage. According to a theory proposed
by Berkeley Lab physicists Marvin Cohen and Steven Louie
(see story on page 40), two-terminal electronic devices
known as diodes should be created at the interface between
two connected but dissimilar tubes, one that acts as a
conductor and the other as a semiconductor. With the help of
a scanning tunneling microscope (STM), Zettl and his group
measured electrical conductivity along the lengths of
connected carbon nanotubes and identified
pentagon-heptagon pair defects (rings of five and seven carbon atoms). These defects
allow electrical current to flow in one direction only, which allows them to function as
rectifier-type diodes.

"What we are seeing is the world's smallest room
temperature rectifier, one that is only a handful
of atoms in size," Zettl said at the time of the
discovery in the fall of 1997.

Another hot application anticipated for nanotubes
is in the area of micro-electromechanical systems
or MEMS. Development of MEMS has been
hampered by frictional wear and tear that occurs
in all devices made from silicon or silicon-based
compounds. Last year, Zettl and members of his
research group custom-engineered carbon
MWNTs into seemingly frictionless bearings and
switches that could prove immensely valuable to
MEMS. Working with an STM inside a
high-resolution transmission electron microscope,
the researchers peeled off the outer layers of a
carbon MWNT while leaving the core nanotube
within fully intact and protruding. They then
demonstrated this core nanotube could be made
to slide in and out of its surrounding jacket like a
well-oiled shaft moving in and out of its sleeve.

"Repeated extension and retraction of telescoping
nanotube segments revealed no wear or fatigue
on the atomic scale," said John Cumings, a
graduate student in Zettl's group. "Hence, these
nanotubes may constitute near perfect,
wear-free surfaces."

The possibility of using telescoped nanotubes as really small and incredibly fast
electromechanical switches arose when it was found that a fully extended core tube
would snap back into its jacket in less than ten billionths of a second.

Explains Cumings, "Because the core nanotube conducts electrically to its housing, an
extended core could bridge a gap between two metals, closing a circuit. When the core
nanotube is retracted, it would open the circuit."

Soon afterwards Zettl and his group, in
collaboration with other groups outside of
Berkeley, found that the electronic
properties of carbon nanotubes are so
extremely sensitive to oxygen that
exposure to air can convert a
semiconducting nanotube into a metallic
conductor.

Working with SWNTs, the researchers
studied both bulk samples and single
isolated tubes, measuring electrical
resistance and thermoelectric power, the
voltage induced by a temperature gradient,
under environmental conditions that
gradually shifted from oxygen to vacuum
and back to oxygen.

"The effects of oxygen exposure became increasingly more irreversible (and have longer
time constants) with decreasing temperature, as expected for a gas adsorption process,"
Zettl says. "In fact, our transport measurements indicate that, once SWNTs have been
exposed to oxygen, it is not possible to fully deoxygenate them at room temperature
even under high vacuum conditions."

Nanotubes need not be made only from pure carbon. Any compound with a propensity for
forming graphite-like sheets is potential nanotube material. Among the most intriguing of
the noncarbon nanotubes are those made from boron nitride (BN), a compound with a
uniform electronic bandgap independent of tube diameter or chirality. This electrical
uniformity means that BN nanotubes can be doped to tune their conductivity much like
silicon. The need has been for a reliable way to make large amounts BN nanotubes.

Zettl and his group have been able to meet this need through the use of an intensely hot
electrical discharge between two boron-rich electrodes in a chamber filled with pure
nitrogen gas. This plasma-arc method yields an abundance of double-walled BN
nanotubes in the gray residue and soot that forms along the chamber walls. These
double-walled nanotubes were found to self-assemble into nanotube bundles or ropes
that can extend several hundred nanometers in length. Also to be found in the gray
residue are BN-coated nanocrystals. Zettl and his group have discovered a means of
hollowing out these crystals through chemical etching to create BN "nanococoons."
Refilling these nanococoons with select atoms or molecules could hold important
technological implications for the chemical and the electrochemical industries.

Transmission electron microscope images of a multiwall
carbon nanotube being shaped. (a) A nanotube in its pristine
form: it contains approximately 37 walls and has an outer
radius of 12.6 nm. (b) A carbon onion has been inadvertently
transferred to the nanotube end from the shaping electrode,
but no attempt has been made to shape the nanotube. (c)(d)
Results of the subsequent peeling and sharpening
processes: the onion has simultaneously been displaced to
a benign position down the tube axis. The shaped, or
'engineered', nanotube in (d) is thick and mechanically rigid
along most of its length (not seen in the image), but tapers
stepwise to a fine sharp tip that is electrically conducting
and ideal for scanning probe microscopy or electron field
emission applications. The final long nanotube segment
contains three walls and has an outer radius of 2.1 nm.

"With nanotubes, we're not seeing the beginning of something that might lead to
something 25 years down the road," says Zettl. "These are things that are crying out to
be exploited in the near future. Nanotubes are on a venture-capital timescale."

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